1. Field of the Invention
This invention relates to graft copolymers. More particularly, the present invention relates to graft copolymers including an olefin backbone polymer and a free radical polymerized polymer grafted thereto made using a borane-containing polyolefin as the backbone polymer.
2. Description of Related Art
Although useful in many commercial applications, polyolefins suffer the major deficiency of poor interaction with other materials. The inert nature of polyolefins significantly limits their end uses, particularly those in which adhesion, dyeability, paintability, printability or compatibility with other functional polymers is paramount. The poor compatibility of polyolefins is further evidenced in their use as coatings where weak adhesion between polyolefins and metal surfaces has not allowed the facile use of this material for the protection of metal. Furthermore, attempts to blend polyolefins with many other non-olefin-based polymers have been unsuccessful because of the incompatibility of the two polymers.
It has been demonstrated that the addition of polar groups to polyolefin polymer structure can improve the adhesion of polyolefin to many substrates, such as metals and glass. In polymer blends, the compatibility of the polymers can be improved by adding a suitable compatibilizer which alters the morphology of these blends. To be successful, it is necessary to reduce the domain sizes for both of the polymers and to increase the interaction between domains.
It is possible to use block or graft copolymers as compatibilizers in such situations as, for example, disclosed in U.S. Pat. No. 4,299,931; Macromolecules 15,370, 1982; Macromolecules 12,131, 1979; J. Polym. Sci, Polym Phys. 18, 2148, 1980; and U.S. Pat. No. 4,174,358. Most block copolymers have been produced by sequential living polymerization processes, particularly anionic polymerization, but such processes are limited to a relatively limited class of monomers. A number of the techniques used to produce these graft or block copolymers are inefficient, resulting in ill-defined products caused by gel formation, backbone degradation, the formation of homopolymers, uncontrolled graft density and molecular weight. These deficiencies are even more pronounced for polyolefins due to their inert nature and difficulties in functionalization reactions involving such polymers. As a consequence, polyolefins have been the most difficult materials to chemically modify, both by functionalization and graft reactions.
This inertness may be overcome to some extent by incorporating polar (functional) groups into polyolefin materials and several techniques for accomplishing this are known. One technique involves oxidizing the polymer backbone by irradiation or contact with a free radical generator (organic peroxide) and then contacting the activated polymer with an unsaturated polar compound such as maleic anhydride. However, such processes can lead to degradation of the polymer backbone during the treatment process. Another technique involves forming copolymers of alpha olefin monomers and copolymerizable monomers containing polar groups.
Among the polyolefins, polypropylene is generally more difficult to functionalize by copolymerization processes. because crystalline polypropylene is made only using Ziegler/Natta catalysts, it is generally difficult to impart functionality to the polymer by copolymerization techniques because polar groups present in the comonomers tend to be reactive with the catalyst system, rendering the catalyst inactive and poisoning it.
U.S. Pat. No. 3,492,277 discloses prereacting a polar monomer such as undecylenic acid, alcohol or amide with an organo aluminum compound thereby rendering such monomers less reactive with Ziegler catalysts. This facilitates their use in forming Ziegler catalyzed alpha olefin copolymers. A similar technique is disclosed in U.S. Pat. No. 4,518,757 wherein copolymers are prepared comprising an alpha olefin and an acid ester comonomer such as methyl-10-undecenoate ester.
More recently, versatile homopolymers and copolymers based on borane-containing monomers have been disclosed. U.S. Pat. Nos. 4,734,472 and 4,751,276 disclose borane-containing monomer material prepared by reacting a diolefin and a dialkyl borane solution. These monomers may be polymerized using Ziegeler-Natta catalysts to form polyborane homopolymers or random copolymers of 1-octene and the borane-containing monomer.
Block or blocky copolymers of propylene and such borane-containing monomers are the subject of copending application Ser. No. 07/637,410, filed on Jan. 4, 1991.
The chemistry involved in the above-mentioned patents and application is the direct polymerization using organoborane-substituted monomers and alphaolefins in Ziegler-Natta processes. The homo- and copolymers containing borane groups are very useful intermediates to prepare a series of functionalized polyolefins.
The essential advantages of this chemistry are (a) the stability of the borane moiety to transition metal catalysts, (b) the solubility of borane compounds in hydrocarbon solvents (hexane and toluene) used in transition metal polymerizations, and (c) the versatility of borane groups which can be transformed to a wide variety of functionalities, as disclosed by H. C. Brown, Organic Synthesis via Boranes; Wiley-Interscience; New York, 1975. Many new functionalized polyolefins with various molecular architectures may be obtained based on this chemistry.
It is also known that trialkyborane in an oxidated state becomes an initiator for the polymerization of a number of vinyl monomers, as disclosed by J. Furukawa et al., J. Polymer Sci, 26, 234, 1957; J. Polymer Sci. 28,227, 1958; Makromol. Chem., 40, 13, 1961; F. J. Welch, J. Polymer Sci. 61,243, 1962; and in U.S. Pat. No. 3,476,727. The polymerization mechanism involves free radical addition reactions. The initiating radicals may be formed from homolysis of peroxyborane or by the redox reaction of the peroxyborane with unoxidized trialkylborane. A major advantage in borane initiators is the ability to initiate polymerization at low temperature. Peroxides and azo initiators when used alone usually require considerable heat input to decompose and thereby to generate free radicals. Elevation of the reaction temperature often causes significant reduction in polymer molecular weight accompanied by the loss of important properties of the polymer.
U.S. Pat. No. 3,141,862, discloses conducting a trialkylborane-initiated free radical polymerization in the presence of a polyolefine polymer. Apparently, the graft reaction by this route was very difficult. The inert nature and insolubility of polyolefin (due to crystallinity) also seems to have hindered the process and resulted in very poor graft efficiency. The reactions shown in the examples of this patent also seem to require a very high concentration of organoborane initiator and monomer and require elevated temperature. The majority of the products are homopolymers or insoluble gel. No information about the molecular structure of the copolymers is given.
Despite the advantages of borane initiators, organoborane-initiated polymerizations tend to be unduly sensitive to the concentration of oxygen in the polymerization system. Too little or too much oxygen results in little or no polymerization. High oxygen causes organoborane to be rapidly transferred to borinates, boronates and borates which are poor initiators at low temperatures. Moreover, polymerization is often inhibited by oxygen. To facilitate the formation of free radicals, U.S. Pat. Nos. 4,167,616 and 4,638,092 disclose that borane containing oligomers and polymers may be used as initiators in such free radical polymerizations. These organoboranes are prepared by the hydroboration of diene monomer or polymers or copolymers. The similar polymeric organoborane adduct, prepared by the hydroboration of 1,4-polybutadiene and 9-borabicyclo(3,3,1)-nonane has also been reported in Macromol. Chem. 178, 2837, 1977. However, no information was provided about the applications of organoborane-containing polyolefin polymers in the preparation of polyolefin graft copolymers.